Sequence alterations that change the meaning of a tRNA in decoding are part of a growing number of post-transcriptional changes collectively known as tRNA editing. Editing can be influenced by the structural context of an editing site and in the case of tRNA can be modulated by posttranscriptional modifications. In trypanosomatids, tRNAs are transcribed in the nucleus, exported to the cytoplasm, and later a subset of cytoplasmic tRNAs is actively imported into the mitochondria. However, before tRNAs can be rendered functional in any cellular compartment, they face many enzymatic reactions including end trimming, intron splicing, tRNA editing, and chemical modification. Some of these processes, for example those involved in trimming of extraneous sequences at the tRNA ends, occur in the nucleus, usually preceding cytoplasmic export. Others, like editing and modification, may occur at any point in the tRNA maturation pathway and in any of the tRNA-containing compartments. Despite much progress made in last few years, it is still not clear how editing and modification pathways are integrated both at the molecular and cellular levels. We have proposed that tRNA editing and modification events can be highly coupled and exploited by some organisms to control gene expression at the level of tRNA specificity. This is especially important in single-cell eukaryotes like trypanosomatid parasites where is well accepted that the bulk of the genetic regulation occurs post-transcriptionally. In this proposal, we have continued our studies on the very unique tRNA editing enzyme of T. brucei but now prompted by our newly solved crystal structure we ask new questions of what makes this enzyme so unique. We also focus on a newly discovered set of methyltransferases, which surprisingly target the editing site providing a wonderful testing ground for our hypothesis of the interrelation of editing and modification and what this coupling may mean in terms of cellular function. Significantly, reconstitution of methylation activity in vitro requires addition of the recombinant editing enzyme a finding that is without precedent in the tRNA editing and modification field. As essential steps in tRNA maturation in trypanosomatids (Leishmania and Trypanosoma), these types of editing and modification events also provide very attractive targets for therapeutic intervention against parasites of very major medical importance. Given the link between tRNA maturation and disease, these studies will further expand our knowledge of the role tRNA takes as a central player in cellular metabolism. !